Programmable spring actuator legs

This proof of concept shows an interesting way of making robotic legs. They are using programmable spring actuators to make these legs function as though they had a “reflex”. The builder pulls the robot along on a leash. As the leg reaches its limit of backward motion, a signal is sent from the shoulder down to the knee that lifts it, causing it to return to the forward position. A signal is also sent to the opposing shoulder to suppress the lift reflex, keeping them from both stepping at the same time. The resulting motion is very organic feeling. We’re curious how this could be implemented in a robot which moves under its own power.

Soggy Pretzels –> The legs appear to only have 2 actuators on each of them. If more were used in a configuration closer to that of a mammal then it probably would have more stability. Also they could probably be setup with some sort of hardware/software neural network to make it run smoother.

I think it looks like a sea turtle being dragged. Though they typically use their underside as a fifth leg.

I went to a talk the creator (Bill Bigge) did at the BuildBrighton hacker space. Its a research project for his PhD. The idea was to make something the home builder could afford. MIT have something like it but its massively expensive.

I did some research on the locomotion in the cat. The stepping reflex is hardcoded in the neural networks in the spine of all mammals and works a lot like this robot. When one leg has reached its hind position, it is brought in front and the other leg is inhibited from lifting up in order to support the weight. This is a fantastic demonstration of this concept. Google «Central pattern generator»

The graph is actually the method of defining the spring damping behavior of the actuator – the X axis is the actuators angle (the axis name was clipped from the image) and the Y axis is force (for the red line) and damping (for the other two lines) you ‘draw’ the spring forces and damping forces with the mouse and download the configuration to the actuator, when it runs it reads its angle and looks up the force value for that angle, reads its velocity and adds in the damping, then applies that force to the output.

This lets you define arbitrary spring damping behaviors that vary across all angles. There are two sets of damping, one for each direction of motion, so you can make things that are highly damped but only in one direction, and only across certain angles.

General note – my prototype actuators are really weak and I could only afford to make four, hence the reason for two legs with two degrees of freedom – you can see in the video that I put a counterweight on the back of the bot to help with the weight. This isn’t a major issue, I just need better motors. The actuators are a bit over-sized as well but I should be able to engineer them to fit into something about the same size and power (and price) of some of the robot servos that are on the market.

You could easily extend this robot to create a hexapod with three degrees of freedom per leg – it would still walk when you pulled it, and you can extend the control system further so it could walk autonomously – I’m also working on embedding a small neural network in each actuator to reproduce some more biological CPG based behaviors.

And also, yes – with the addition of a balance sensor you could use them to make your own miniature version of big dog, but for a few hundred dollars.